How Does A Piano Work?

There's a musical and mechanical side to your question.

On the musical side, it's made up of scales of which chords can be created from.  Melodies are played using scales.

On the mechanical side, the piano is a stringed instrument.  When each key is depressed, a hammer strikes the corresponding string to make the sound.

You can learn much more about how the piano works as an instrument at:

www.thepianobyear.com